Wind turbine generators require redundant braking systems in order to satisfy safety requirements, e.g., as specified by the insurer Germanischer Lloyd. A first brake system conventionally includes a disk brake capable of stopping a turbine against full wind torque and either a hydraulic accumulator or stored spring energy to enable operation in the event of a power failure. A second brake system conventionally uses aerodynamic braking to pitch the turbine blades into a feathered position and includes a stored energy source so that blade pitch can occur after a loss of power in a utility grid. Blade pitch has traditionally been accomplished on commercial wind turbines with a hydraulic ram and rotating coupling arrangement which can be readily backed up with a hydraulic accumulator.
Several recent wind turbine designs have incorporated electric servo pitch actuators to eliminate maintenance problems associated with hydraulic rams and accumulators. These systems require battery energy storage to enable blade pitch after utility grid power loss. The emergency batteries, however, have limitations related to size, weight, cost, and reliability. In particular the relatively uncontrolled temperatures in a wind turbine severely limit the life expectancy of the batteries.